The present invention relates to a device for controlling the combustion process in a combustion engine. The invention especially relates to such a device for reducing especially soot emissions but also carbon monoxide and hydrocarbon in combustion engines in which the fuel/cylinder gas mixture is ignited by compression heat generated in the cylinder.
Soot particles (or particulates) are a product which, during combustion, can both be formed and subsequently oxidized into carbon dioxide (CO2). The quantity of soot particles measured in the exhaust gases is the net difference between formed soot and oxidized soot. The process is very complicated. Combustion with fuel-rich, fuel/air mixture with poor mixing at high temperature produces high soot formation. If the formed soot particles can be brought together with oxidizing substances such as oxygen atoms (O), oxygen molecules (O2), hydroxide (OH) at sufficiently high temperature for a good oxidation rate, then a greater part of the soot particles can be oxidized. In a diesel engine, the oxidation process is considered to be in the same order of magnitude as the formation, which means that net soot production is the difference between formed quantity of soot and oxidized quantity of soot. The net emission of soot can therefore be influenced firstly by reducing the formation of soot and secondly by increasing the oxidation of soot. Carbon monoxide emissions (CO) and hydrocarbon emissions (HC) are normally very low from a diesel engine. Yet the percentages can rise if unburnt fuel ends up in relatively cool regions. Such regions are, in particular, zones with intense cooling located close to the cylinder wall. Another example is cavities between piston and cylinder lining.
Nitrogen oxides (NOx) are formed from the nitrogen content in the air in a thermal process which has a strong temperature dependency and depends on the size of the heated-up volume and the duration of the process.
A combustion process in which the fuel is injected directly into the cylinder and is ignited by increased temperature and pressure in the cylinder is generally referred to as the diesel process. When the fuel is ignited in the cylinder, combustion gases present in the cylinder undergo turbulent mixing with the burning fuel, so that a mixture-controlled diffusion flame is formed. The combustion of the fuel/gas mixture in the cylinder gives rise to heat generation, which causes the gas in the cylinder to expand and which hence causes the piston to move in the cylinder. Depending on a number of parameters, such as the injection pressure of the fuel, the quantity of exhaust gases recirculated to the cylinder, the time of injection of the fuel and the turbulence prevailing in the cylinder, different efficiency and engine emission values are obtained.
Below follows two examples of state of the art arrangements attempting to lower both soot and NOx-emissions by controlling the flame, and trying to brake the well known “trade off” between soot emissions and nitrogen oxide emissions, which is typical of the diesel engine, and which “trade-off” is difficult to influence. The majority of measures which reduce soot emissions increase the nitrogen oxide emissions.
EP1216347 shows an arrangement for controlling the combustion process in a combustion engine by controlling the combustion flame, with the purpose to decrease soot and NOx emissions. The fuel is injected into the combustion chamber with a sufficiently high kinetic energy (high injection pressure) so as to supply kinetic energy to the spray in such way that a spray-internal mixing process and a large-scale global mixing process between fuel and cylinder gas is achieved, thus keeping the soot emissions below a selected level. A proportion of recirculated exhaust gas is selected such that the nitrogen oxide emissions are kept below a selected level.
U.S. Pat. No. 6,732,703 shows an arrangement for minimizing NOx emissions and soot particulates. Here, the fuel spray hits inner bowl floor section during injection in order to cool down the combustion and thereby decreasing the creation of NOx. The fuel is injected with high pressure and the piston is shaped to maintain the momentum in the spray plume/flame and fuel/air mixture so that good mixing of available oxygen and soot occurs late in the combustion process. A lot of the momentum is lost when the spray plume hits the inner bowl floor section and when two adjacent flames hit each other during circumferential flame progress.
U.S. Pat. No. 5,215,052 discloses an arrangement for improved mixture of fuel/air and decreased flow loss of flame expansion in a circumferential direction in the combustion space.
This is done by providing a piston with a shallow piston recess, and depressions in the recess bottom so that they have a corrugated shape in relation to the circumferential direction of the piston recess, and mainly in a plane perpendicular to the reciprocal movement of the piston. Still a lot of momentum will be lost when the flame progresses in a circumferential direction that is perpendicular to the reciprocal movement of the piston. This arrangement is not adapted for enhanced late soot oxidation.
JP59010733 discloses a combustion chamber 8 with protrusions 10 at the top of and inside the piston bowl. There is one protrusion for each spray. Each fuel spray aims at its protrusion as to increase the flow speed of the spray along the circumferential wall of the piston bowl when it is dashed against the protrusion. A lot of the momentum is lost especially when adjacent flames hit each other during circumferential flame progress.
Due to coming future emission legislation for combustion engines there is a need to further lower the soot emission levels in order to meet coming demands.
It is, therefore, desirable to overcome the deficiencies of the prior art and to provide an internal combustion engine containing a combustion chamber arrangement designed to reduce undesirable soot emissions sufficiently to meet new regulated limits. Thus, it is desirable to minimize the amount of soot by promoting efficient flame recirculation and thereby “saving” mixing energy to the final oxidation of soot and remaining fuel. The soot reduction is especially important for fuels such as for example diesel. It is also desirable to contribute to the reduction of carbon monoxide (CO) emissions and hydrocarbon (HC) emissions. The reduction of CO and HC becomes especially important for fuels such as for example DME (dimethyl ether).
It is also desirable to provide an engine wherein the shape, position and dimensions of various features of the combustion chamber arrangement, cause the spray/flame to impinge upon and contact the piston bowl surface in the outer bowl section and in order to optimize preservation of kinetic energy in flame movements, mainly directed in a plane perpendicular to the reciprocal movement of the piston.
It is also desirable to provide a diesel engine capable of operate with significant soot emission improvements compared to e.g. an US02-engine, while also satisfying mechanical design constraints for a commercially acceptable engine.
It is also desirable to provide an engine including a combustion chamber arrangement having dimensions and dimensional relationships to ensure oxidation of sufficient amount of soot during combustion to minimize soot available for discharge to the exhaust system. This can be done without increasing the creation of NOx.
According to an aspect of the present invention, an engine with a combustion chamber is provided, comprising: an engine body including an engine cylinder, a cylinder head forming an inner surface of the combustion chamber and at least one intake port; a piston positioned for reciprocal movement in said engine cylinder between a bottom dead center position and a top dead center position, said piston including a piston crown comprising an upper surface facing the combustion chamber, said piston crown containing a piston bowl formed by an outwardly opening cavity, said piston bowl comprising a projecting portion having a distal end and an inner bowl floor section extending downwardly at a positive inner bowl floor angle from a plane perpendicular to an axis of reciprocation of the piston, said piston bowl further comprising an outwardly flared outer bowl section having a concave curvilinear shape in cross section; an injector mounted on the engine body adjacent said projecting portion of said piston bowl to inject fuel into the combustion chamber with high injection pressure, said injector comprising a plurality of orifices arranged to form fuel spray plumes, which during progress become ignited flames that impinge within predetermined impingement areas on said outer bowl section. The aspect of the invention is characterized in that said impingement areas are in the outer bowl section during most of the injection and in that substantially half way between said impingement areas in the outer bowl section and in a plane perpendicular to said reciprocal movement are arranged a first type of protrusions protruding into the combustion chamber and having a smooth form adapted for preserving kinetic energy in the flame and for redirecting circumferential flame progress mainly towards a center axis of the piston with minimal flame-to-flame interaction.
According to one embodiment of an aspect of the invention a second type of protrusions are arranged in said impingement area. Said second type of protrusions are adapted for redirecting flame progress directed towards the impingement area mainly into a circumferential flame progress direction in a plane substantially perpendicular to said reciprocal movement and with minimal flame-to-piston wall interaction and minimal kinetic energy loss.
In a further developed embodiment of an aspect of the invention said protrusions has a shape of a longitudinal ridge that extends only in the outer bowl area in a plane substantially parallel to said reciprocal movement of said piston. In another embodiment a cross-section, perpendicular to the extension of said ridge, of a top of said ridge is formed with a curved shape with a average radius that is at least 1/20 of a piston bowl radius of said piston. According to another embodiment said first type of protrusions are protruding more into the combustion chamber compared to said second type of protrusions.
Said internal combustion engine can have a first impingement in said impingement area when start of injection and a second impingement point in said impingement area when end of injection. Said ridge can be extended at least from a first position arranged in a first plane that is common for said first impingement point and said first position, and up to a second position arranged in a second plane that is common for said second impingement point and said second position. Said first and second planes are perpendicular to the reciprocal movement of said piston.
In one further preferred embodiment of an aspect of the invention said central axis is arranged to impinge said outer bowl section during the whole injection.
In another further preferred embodiment of an aspect of the invention said intake port is formed in the cylinder head for directing intake air into the combustion chamber with no or low swirling effect during operation. In a further preferred embodiment of the invention said swirling effect has a swirl ratio in the range of 0.0 to 0.7.
In another further preferred embodiment of an aspect of the invention a geometry of the inner bowl floor section in relation to the spray axis is arranged in such a way so that there is enough volume and distance between the inner bowl floor section and the spray axis (30) so that disturbing contact between the unignited nozzle near portion of the spray and the inner bowl section is avoided.
In another further preferred embodiment of an aspect of the invention said injected fuel, when injected, is arranged to form a mixture with said intake air in said combustion chamber, and that said mixture self ignites when compressed by said piston.
In another further preferred embodiment of an aspect of the invention said engine is arranged to add a predetermined portion of re-circulated exhaust gas to said intake air, said portion being adapted so that nitrogen oxide emissions emerging from said combustion are kept below a selected low level.